A-list interior designer Stuart Rattle and Michael O'Neill were known as the "A Gays".

The handsome couple lived and worked together in Rattle's high-end interior design business. Sought after by the wealthy and connected, they circulated with the "A Gays", a good-looking and social clique from the worlds of art, design, fashion, hairdressing, food and wine.

They were surrounded by beautiful antiques and fabrics, they lived in South Yarra and had a prized getaway, Rattle's beloved Musk Farm, near Daylesford. They travelled in style – in a Range Rover – flew first-class. To some, they were the perfect, smiling couple.

Michael O'Neill (left) and Stuart Rattle. Photo: Supplied

Now O'Neill is staring at the prospect of bedding down in jail after pleading guilty to murdering Rattle and arson. O'Neill, 48, pleaded guilty to murdering Rattle, 53, by hitting him over the head with a saucepan and strangling him with a nylon dog lead on December 4. O'Neill also pleaded guilty to arson by setting fire to their South Yarra home four days later while Rattle lay dead in his bed.

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How did the lives of two men who loved each other and were partners for 16 years end in murder?

Those close to the duo have speculated about a business that was in disarray and the emotional state of O'Neill. While Rattle was the face of the business, O'Neill grew resentful of being the office boy and, from what friends say, an inept one who was out of his depth and a pathological liar about his background and most other things.

People called O'Neill "The Talented Mr Ripley" and he was an exasperating one because no one could tell if he was lying or telling the truth.

Bespoke furniture maker Kim Moir knew Rattle for 22 years and O'Neill for 16, saying: "Michael has always been staggeringly unusual and frustrating but their relationship was always very good."

Mr Moir said: "Michael was a terrible, terrible administrator of the office and Stuart knew it. Everyone had a go at Stuart about it and he said 'What else can I do?' He told me: 'I've sacked him so many times and he won't leave'."

The situation was so dire that Rattle put on an extra staff member to keep an eye on O'Neill without him knowing. Customers would ask O'Neill about their table and he'd say it would be delivered next week but Mr Moir hadn't even received the drawings. Customers would ring Mr Moir to say O'Neill told them a truck was coming next week but the furniture-maker said: "What table? I hadn't even quoted, there was no deposit, no order, there was no truck."

Naturally, Rattle's reputation was on the line. Mr Moir said: "Stuart was tearing his hair out for years and tried different systems." There was a day book that O'Neill was supposed to fill out to indicate where he was, what he was doing, and who he was with. "Nothing worked, he wouldn't answer the phones. This was going on for four, five or six years. It was getting worse and worse and worse."

A close friend of the duo said: "On the day of the murder, Michael had screwed up huge with a very well-known client of Stuart's in front of everyone who was down at Portsea. Michael had f--ked up big in the business and just told so many lies. He lied about where he came from with a complete fantasy school."

Armadale antiques dealer Graham Geddes knew Rattle when he was 16 and exploring his creativity by making frocks and curtains, whereas O'Neill was a former waiter who worked at the Queenscliff Hotel of the late Mietta O'Donnell, Cafe Latte and Caffe e Cucina. Mr Geddes believes O'Neill was unable to cope with the complexities of running a business and knew his inadequacies but was incapable of rectifying them.

A source within their social circle, who declined to be named, said there was speculation that the duo was in debt due to their first-class lifestyle. "They had been fighting for years and the relationship had been breaking down."

Long-term couples are familiar with this scenario but what separated Rattle and O'Neill was this stark character assessment: "Michael wanted to be Stuart and was always very jealous of him. He was obsessive about it. Stuart was tiring of it."

This makes sense because after O'Neill killed Rattle, he started acting like him. He rang clients, saying Rattle wasn't well and he was taking over, declaring: "I'll be able to handle everything." Then after the fire, he rang more clients, saying: "I have everything in hand."

Mr Geddes said: "Michael became a little intoxicated by the exuberance of himself. He thought he could take on a lot of tasks. I believe he was in over his head. As the business grew in power, the complications of running the business would have caused stress for Michael. He was failing and that would have caused anxiety and insecurity. And that would have been reflected in his relationship with Stuart."

As for the duo's grand lifestyle, Mr Geddes said: "As things became tighter, some people in this business get a little carried away with themselves and live the life of their clients. A lot of clients are extremely wealthy. Basically, we are just there to serve."

As for the stressors of running a business together as life partners, Mr Geddes said: "Stuart was quite moody and there would have been times they would have been bitchy to each other. This would have been exacerbated by the lack of management of the business. Stuart became aware of it and couldn't be bothered with it because he was more interested in looking at the colour of drapes."

While it is abhorrent for people without murder in their veins to live with a dead body in the same apartment for four days, O'Neill carried on as usual. Was it grief, shock or the demeanour of a monster?

Mr Moir and others were stunned that O'Neill used Rattle's phone as if nothing was wrong. "He was texting me from Stuart's phone as if he was Stuart," he said. Also staggering is the couple's connection to Rattle's Musk Farm and their link to the Friends of Wombat Hill Botanic Gardens. With Rattle as vice-president and O'Neill as treasurer – ironic given his fiscal ineptitude – the duo hosted an "open garden" at the farm last November and raised more than $125,000 for the botanic gardens restoration works.

O'Neill killed his partner on Wednesday, December 4, and on the Friday he went to Daylesford for drinks to celebrate the success of the open garden, telling dignitaries that Rattle was unwell but he would attend.

At a crime scene, certain things don't make sense when the personality of the deceased is considered. On the night O'Neill burnt the apartment, he admitted to kicking over candles next to the curtains. Those close to Rattle knew he didn't light candles – they were only for decoration. Mr Moir said: "Stuart never used candles. You never lit them because it's too dangerous."

Friends of the duo who rallied around O'Neill as the "grieving widow" are still coming to terms with the horrific death. "We believe it was an awful crime of passion," one friend said. "It's so sad. We don't know, we really don't know why. No relationship is perfect and you never know what goes on behind closed doors. Michael had a whole backstory that he wanted to keep hidden. Stuart may have found out."

For those who think they can get away with murder, one friend said what a lot of people are thinking: "You could have murdered someone a lot easier." As for O'Neill's friendship: "I feel sorry for him. Obviously he was tormented."